Method and apparatus for producing MIIIN columns and MIIIN materials grown thereon

ABSTRACT

A method utilizes sputter transport techniques to produce arrays or layers of self-forming, self-oriented columnar structures characterized as discrete, single-crystal Group III nitride posts or columns on various substrates. The columnar structure is formed in a single growth step, and therefore does not require processing steps for depositing, patterning, and etching growth masks. A Group III metal source vapor is produced by sputtering a target, for combination with nitrogen supplied from a nitrogen-containing source gas. The III/V ratio is adjusted or controlled to create a Group III metal-rich environment within the reaction chamber conducive to preferential column growth. The reactant vapor species are deposited on the growth surface to produce single-crystal M III N columns thereon. The columns can be employed as a strain-relieving platform for the growth of continuous, low defect-density, bulk materials. Additionally, the growth conditions can be readjusted to effect columnar epitaxial overgrowth, wherein coalescence of the Group III nitride material occurs at the tops of the columns, thereby forming a substantially continuous layer upon which additional layers can be deposited. The intervening presence of the column structure mitigates thermal mismatch stress between substrates, films, or other layers above and below the columns. A high deposition rate sputter method utilizing a non-thermionic electron/plasma injector assembly is provided to carrying out one or more of the growth steps.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication Ser. Nos. 60/250,297, filed Nov. 30, 2000; and 60/250,337,filed Nov. 30, 2000; the disclosures of which are incorporated herein byreference in their entireties.

TECHNICAL FIELD

The present invention is generally directed to the production of GroupIII metal nitride materials for use as free-standing articles as well assubstrates for further processes and/or microelectronic andoptoelectronic devices. In particular, the present invention is directedto the production of low-defect density single-crystal materials grownfrom a strain-relieving layer of single-crystal columns, utilizingenhanced sputtering techniques.

BACKGROUND

A wide variety of techniques exist for depositing thin films ontosubstrates in order to achieve desirable properties which are eitherdifferent from, similar to, or superior to the properties of thesubstrates themselves. Thin films are employed in many kinds of optical,electrical, magnetic, chemical, mechanical and thermal applications.Optical applications include reflective/anti-reflective coatings,interference filters, memory storage in compact disc form, andwaveguides. Electrical applications include insulating, conducting andsemiconductor devices, as well as piezoelectric drivers. Magneticapplications include memory discs. Chemical applications includebarriers to diffusion or alloying (e.g., galling), protection againstoxidation or corrosion, and gas or liquid sensors. Mechanicalapplications include tribological (wear-resistant) coatings, materialshaving desirable hardness or adhesion properties, and micromechanics.Thermal applications include barrier layers and heat sinks.

Bulk materials can be used as substrates upon which microelectronic andoptical devices are fabricated. Wide bandgap semiconductor materialssuch as gallium nitride, aluminum nitride, indium nitride and theiralloys are being studied for their potential application inmicroelectronics and opto-electronics. These materials are particularlywell suited for short wavelength optical applications, such as green,blue and UV light emitting devices (LEDs and LDs), and visible andsolar-blind UV detectors. The use of UV or blue GaN-based LEDs makespossible the fabrication of solid state white light sources, with higherefficiencies and lifetimes 10 to 100 times longer than conventionalsources. Additionally, GaN has a region of negative differentialmobility with a high peak electron velocity and high-saturated velocity,which can be used for fabricating high-speed switching and microwavecomponents. P-type doping of GaN and AlGaN with relatively high holeconcentrations is now readily achieved, and ohmic and Schottky contactshave been characterized for n- and p-type materials. Thus, many of theabove devices have or potentially have large, technologically importantmarkets. Such markets include display technology, optical storagetechnology, and space-based communications and detection systems. Otherapplications include high temperature microelectronics, opto-electronicdevices, piezoelectric and acousto-optic modulators, negative-electronaffinity devices and radiation/EMP hard devices for military and spaceuses.

Attempts to grow low-defect density gallium nitride (GaN) thin filmsheteroepitaxially on substrates such as sapphire and silicon carbide(SiC) have had limited success. GaN materials heteroepitaxially grown onthese substrates suffer from large concentrations of threading defects,typically on the order of 10⁻⁸-10⁻¹⁰ cm⁻², due to the large latticemismatch between the film and substrate. Threading defects increaseleakage currents in diode and FET structures and act as a significantsource of noise in photodetectors. As a result, the operation of highperformance devices, such as high-speed, high-sensitivity UVphotodetectors, and high power, high frequency microelectronic devices,is presently limited. Buffer layers of AlN, GaN, and other materialshave been used to reduce the lattice mismatch. However, threadingdefects and low angle grain boundaries remain in the films. Differencesbetween the film and substrate thermal expansion coefficients alsoresult in stresses in the films.

Accordingly, homoepitaxial growth of GaN thin films on bulk GaNsubstrates is of great interest. The use of GaN substrates wouldeliminate the problems due to lattice mismatch and thermal expansionmismatch. Unfortunately, the availability of GaN substrates has beenlimited due to conventional processing capabilities. This problem hashindered the development of devices based on GaN and related nitridesemiconductors. Several obstacles exist to the successful manufacturingand commercializing of high device-quality Group III nitride-basedmaterials, whether in bulk, single-crystal, polycrystalline or epitaxialform, for electronics and other applications. These obstacles generallyinclude cost, reproducibility, and purity.

For instance, gallium nitride has a high equilibrium vapor pressure ofnitrogen that results in its decomposition at elevated temperatures. Thesolubility of nitrogen in gallium metal at room temperature and pressureis very low. As a result, conventional crystal growth methods to produceGaN are not practical. This has led to the development of severalalternate bulk growth methods, including high-temperature, high-pressure(15 kbar) solution growth, evaporation, and sublimation.

Currently, aluminum nitride and gallium nitride exist only aspolycrystalline or powder forms, or in thin films. Polycrystalline bulkaluminum nitride can be manufactured using powder processing techniques.This process has not yielded semiconductor-grade single crystalmaterial. Formidable problems are associated with such techniques,beginning with the production of pure aluminum nitride powders and thenthe sintering of oxygen-free and defect-free aluminum nitride. Some ofthese problems include the production of both high-purity and uniformparticle-size powders. The highest purity powders can contain up to 1%of oxygen and binders, such as Y₂O₃, that are needed to produce aluminumnitride with a high density. Therefore, high density is achievable atthe expense of contamination. Sintering of these aluminum nitridepowders is also a difficult process. The covalent nature of aluminumnitride prevents densification of pure aluminum nitride at lowtemperatures. Aluminum nitride decomposes at high temperatures, such asabove 1600° C., thereby preventing densification. Hence, costlysintering aids such as high pressures and impurities are required forproducing high-density material. Other problems associated with powderprocessing of aluminum nitride include maintaining the purity andintegrity of the powder, controlling the environment at high sinteringtemperatures, and the production of defect-free parts. Aluminum nitrideis very difficult to manufacture using powder processing techniqueswithout introducing contamination that will have adverse effects on theoptical and thermal properties of the material. These impurities can bepresent in the crystalline lattice structure, and can migrate to thegrain boundaries during sintering, causing the infrared absorbance to behigh.

Various masking techniques have been explored in conjunction withlateral epitaxial overgrowth (LEO) and selective area growth (SAG)techniques in search of improved methods for fabricating low-defectdensity gallium nitride crystal layers. For example, U.S. Pat. No.6,153,010 discloses a method for growing nitride semiconductor crystals.A nitride buffer layer is first grown on a substrate using gaseous GroupIII element and nitrogen sources (e.g., MOVPE, MBE, or HVPE). Using avapor-phase technique and photolithography, an oxide selective growthmask is formed on the underlayer. The mask is configured as discretestripes so that areas of the buffer layer remain exposed. Nitridesemiconductor material portions are then grown on these exposed areasusing gaseous Group III element and nitrogen sources. When such growthexceeds the upper ends of the mask stripes, the semiconductor materialgrows laterally on the mask stripes. Continued vertical growth resultsin the material portions combining to form an integral nitridesemiconductor crystal. This process can be repeated to grow a secondintegral nitride semiconductor crystal from a second selective growthmask formed on the first integral nitride semiconductor crystal. Inanother disclosed embodiment, a nitride semiconductor layer is depositedon the substrate/buffer layer, and recesses are etched into thesemiconductor layer. A first growth control mask is formed on theremaining top surfaces of the semiconductor layer, and a second growthcontrol mask is formed in the respective bottom surfaces of therecesses. Nitride semiconductor material portions are then grown in therecesses by a vapor-phase technique that relies on gaseous Group IIIelement and nitrogen sources. This is followed by lateral growth and theformation of an integral nitride semiconductor crystal.

In view of the state of the art as described hereinabove, it would beadvantageous to provide a method and apparatus for growingdevice-quality nitride material without the need for masking and etchingprocedures.

As disclosed hereinbelow, it has now been discovered that enhancedsputtering techniques, which are physical vapor deposition (PVD)techniques, can be feasibly utilized to produce low-defect density GroupIII metal nitride materials of bulk thickness and of device-qualitycrystal, and thus utilized as part of methods of the invention disclosedhereinbelow. Magnetron sputtering is traditionally associated with thinfilm deposition. An advantage of sputter synthesis is that high puritycompounds can be formed directly from the high purity source materials.Moreover, the synthesis can be achieved under highly controlledconditions. Nitrogen and Group III metals such as aluminum are readilyavailable, from multiple sources, in ultra-high purity grades (e.g.,99.9999%) for the microelectronics industry. Sputter synthesis iscurrently the process that most effectively eliminates hydrogen from thebulk, since the sputter environment is controllable to ultra-high vacuumconditions. Through sputter synthesis of Group III nitrides, it ispossible to obtain materials that have properties near the bulkproperties. Since this takes place under ultra-high vacuum conditions,hydrogen and oxygen can be eliminated from the material. Reactivesputtering has the advantage of producing high purity materials withhigh densities, and ease of fabrication of quality crystalline material.

However, traditional magnetron sputtering has several drawbacks, whichhas made it very difficult to produce bulk materials. These drawbacksinclude unwanted target reactions, transport limitations, and low growthrates. During reactive magnetron sputtering, micro-arcs can occur on thecathode surface which cause imperfections in the deposited material.Another problem associated with this process is the “disappearing anode”effect, in which the entire anode becomes covered by randomly growninsulating layers. Also related to this process is the problematicformation of an insulating nitride layer on the target surface thatincreases the impedance of the cathode until the target becomes“poisoned” or completely insulating. This results in a drastic decreasein deposition rates to almost zero when the target becomes too nitridedto operate. Materials transport can also be a problem in bulk crystalgrowth using magnetron sputtering since there can be a significant lossof material to the sidewalls.

The present invention is provided to address these and other problemsassociated with the growth of thin films and bulk materials.

DISCLOSURE OF THE INVENTION

The present invention provides a method that utilizes sputter transporttechniques to produce arrays or layers of self-forming, self-orientedcolumnar structures characterized as discrete, single-crystal Group IIInitride posts or columns on various substrates. This columnar structureis formed in a single growth step, and therefore does not requireprocessing steps for depositing, patterning, and etching growth masks.Characterization of such structures reveals discrete Group III nitridecolumns exhibiting a hexagonal crystal habit. Pursuant to the invention,the columns can be grown several tens of microns thick while maintainingthe average column size. In some embodiments of the invention, thecolumns are advantageously employed as a starting structure for thegrowth of continuous, low defect-density, bulk materials. In a processtermed herein as columnar epitaxial overgrowth (CEO), the highlyoriented columnar structure is grown on a substrate or template materialand, subsequently, the growth conditions are changed to effectcoalescence of the Group III nitride material at the tops of thecolumns. As a result, a bulk crystal is grown on the columnar basestructure in a single growth run. The bulk crystal has a distinctadvantage over the conventional growth of layers directly on asubstrate, in that the thermal mismatch stress between the bulk crystaland the substrate is mitigated by the intervening presence of the columnstructure. This, in turn, greatly reduces the degree of cracking andbowing in the resulting multi-layered structure, and eases post-growthprocessing. The crystal quality of the bulk crystal grown on the columnsis characterized as being superior to or at least equivalent to GroupIII nitride layers grown directly on a substrate such as sapphire.

According to one method of the present invention, single-crystalM^(III)N columns are produced. A template material having anepitaxial-initiating growth surface is provided. A Group III metaltarget in a plasma-enhanced reaction chamber is sputtered to produce aGroup III metal source vapor. A nitrogen-containing gas is introducedinto the reaction chamber. The III/V ratio of Group III metal sourcevapor to nitrogen is adjusted to create a Group III metal-richenvironment within the reaction chamber conducive to preferential columngrowth. The Group III metal source vapor is combined with thenitrogen-containing gas to produce a reactant vapor species comprisingGroup III metal and nitrogen. The reactant vapor species are depositedon the growth surface to produce single-crystal M^(III)N columnsthereon.

The growth temperature is within a range of approximately 400° C. toapproximately 1200° C. and, more preferably, within a range ofapproximately 600° C. to approximately 1000° C.

In one aspect of the method, the reactant vapor species are depositeddirectly on the growth surface. In an alternative aspect, anintermediate layer is deposited on the growth surface prior todepositing the reactant vapor species. Preferably, the columns have anaverage thickness that is greater than that of the intermediate layer.Additionally, the columns have an defect density lower than that of theintermediate layer and the template material.

Preferably, the columns grown according to the invention have an averagethickness of approximately 0.5 micron or greater, and a defect densityof 10⁸ defects per cm² or less.

According to another method of the invention, the III/V ratio isreadjusted to create an environment within the reaction chamber that isconducive to columnar epitaxial overgrowth, such that continueddeposition of the reactant species on the growing M^(III)N columnsresults in the upper regions of the columns coalescing so as to form asubstantially continuous, single-crystal M^(III)N layer. Preferably,this M^(III)N layer has a lower defect density than that of the columns.

According to yet another method of the invention, M^(III)N layer on thecolumns is used as a buffer or transition layer for the growth of abulk, single-crystal homoepitaxial or heteroepitaxial M^(III)N article.

A broad range of techniques can be employed for growing any intermediatelayer or M^(III)N article according to the methods of the invention,including physical vapor deposition, sputtering, molecular beam epitaxy,atmospheric chemical vapor deposition, low pressure chemical vapordeposition, plasma-enhanced chemical vapor deposition, metallorganicchemical vapor deposition, evaporation, sublimation, and hydride vaporphase epitaxy.

According to one embodiment of the invention, a single-crystal column isproduced according to methods described herein. Preferably, the columnhas a height of approximately 0.5 micron or greater, a lateral dimensionof approximately 0.05 micron or greater, and a defect density ofapproximately 10⁸ defects per cm² or less.

According to yet another method of the invention, a single-crystalM^(III)N article is produced. A template material having anepitaxial-initiating growth surface is provided. A Group III metaltarget is sputtered in a plasma-enhanced reaction chamber to produce aGroup III metal source vapor. A nitrogen-containing gas is introducedinto the reaction chamber. The III/V ratio of Group III metal sourcevapor to nitrogen-containing gas is adjusted to create a Group IIImetal-rich environment within the reaction chamber conducive topreferential column growth. The Group III metal source vapor is combinedwith the nitrogen-containing gas to produce a reactant vapor speciescomprising Group III metal and nitrogen. The reactant vapor species isdeposited on the growth surface to produce single-crystal M^(III)Ncolumns thereon. A bulk, single-crystal M^(III)N article is grown on theM^(III)N columns.

Preferably, the M^(III)N article has a thickness ranging fromapproximately 1 micron to greater than 1 mm, and a diameter ranging fromapproximately 0.5 inch to approximately 12 inches.

According to still another method of the invention, the M^(III)N articleis released to provide a free-standing, single-crystal M^(III)N article.The removal technique utilized can be, for example, of polishing,chemomechanical polishing, laser-induced liftoff, cleaving, wet etching,and dry etching.

According to a further method of the invention, a wafer is cut from theM^(III)N article. A surface of the wafer can be prepared for epitaxialgrowth, such as by polishing, and an epitaxial layer can then be grownon the prepared surface.

According to a still further method of the invention, a device orcomponent is fabricated on the M^(III)N article.

According to another embodiment of the invention, a bulk, single-crystalM^(III)N article is produced by one of the above-described methods.Preferably, the M^(III)N article has a diameter ranging fromapproximately 0.5 inch to approximately 12 inches and a thickness ofapproximately 50 microns or greater.

According to yet another embodiment of the invention, the article isproduced in the form of a wafer having a thickness ranging fromapproximately 50 microns to approximately 1 mm.

According to still another embodiment of the invention, the article isproduced in the form of a boule having a diameter of approximately 2inches or greater and a thickness ranging from approximately 1 mm togreater than approximately 100 mm.

Methods of the present invention can be implemented by providing a novelsputter material transport device to enhance thin-film and bulk materialmanufacturing processes. The novel transport device is capable ofultra-high deposition and growth rates, making it feasible for growingthick material and increasing throughput in manufacturing processes. Thetransport device can be used both to grow bulk crystalline materials andto deposit thin films and epitaxial layers onto bulk substrates.Generally, as compared to other sputter processes, the transport deviceof the present invention has the advantages of lowered processingpressure, higher deposition rates, higher ionization efficiency, and acontrolled processing environment with no contamination. The transportdevice utilizes an enhanced sputtering process to rapidly deposit bothmetallic and dielectric materials. This enhancement allows the processto overcome the limitations of conventional PVD techniques.

The transport device according to the present invention can achievegrowth rates in excess of ten times those achieved by any other directdeposition process. As currently tested, the device is capable ofdepositing single or polycrystalline material at a rate in excess ofapproximately 60 μm/hr. This high deposition rate allows for highthroughput capabilities and the possibility of manufacturing bulkmaterials in short time periods. The device has increased growth ratesdue to the very high ionization efficiencies, which enhances thesputtering process without “poisoning” the sputtering material. Theability to deposit material at high deposition rates will have manycommercial applications, including high-throughput manufacturingprocesses of thick films of exotic materials. Moreover, high-qualitymaterial can be deposited in a cost-effective manner. It is alsoprojected that the device will aid in the commercialization of bulkdielectric and semiconductor materials and will have numerousapplications to other materials.

The transport device according to the invention surpasses presenttechnology by offering a non-contaminating method, as implemented by atriode sputtering device, to increase the ionization efficiency andhence the overall deposition rate. The device also has the advantage ofa cooler operating temperature than a thermionic hollow cathodeconfiguration, allowing the injector means of the device to be composedof low-temperature materials, and thus can apply to a broad range ofmaterials as compared to conventional processes. The transport devicecan increase the deposition rate of the target material and lower thesputtering pressure, thereby enabling a line-of-sight depositionprocess.

The transport device is capable of growing bulk material such asaluminum nitride and other Group III nitrides and also is capable ofdepositing metal in deep trenches for the semiconductor industry.

According to the present invention, the transport device includes amagnetron source and a non-thermionic electron (or, in effect, a plasma)injector assembly to enhance magnetron plasma. Preferably, theelectron/plasma injector assembly is disposed just below the surface ofa cathode target material, and includes a plurality of non-thermionic,hollow cathode-type injector devices for injecting electrons into amagnetic field produced by a magnetron source. The injector can bescaled in a variety of configurations (e.g., circular or linear) toaccommodate various magnetron shapes. When provided in the form of acircular ring, the injector includes multiple hollow cathodes locatedaround the inner diameter of the ring.

The transport device constitutes an improvement over previouslydeveloped hollow cathode enhanced magnetron sputtering devices, in thatthe device of the present invention is a non-thermionic electron emitteroperating as a “cold” plasma source and can be composed of the samematerial as its sputtering target. The injector can be manufactured outof high-purity metals (e.g., 99.9999%), thereby eliminating a source ofcontamination in the growing film. The addition of the injector to themagnetron sputtering process allows higher deposition rates as comparedto rates previously achieved by conventional magnetron sputteringdevices. Moreover, the transport device takes advantage of the hollowcathode effect by injecting electrons and plasma into the magnetic fieldto increase plasma densities without the contamination problemassociated with a traditional, thermionic-emitting tantalum tip. Asdisclosed above, the transport device is further characterized by adecreased operating pressure and an increased ionization rate overconventional magnetron sputtering.

Therefore, according to another method of the present invention,single-crystal M^(III)N columns are produced by using a sputteringapparatus comprising a non-thermionic assembly disposed in a reactionchamber to produce a Group III metal source vapor electron/plasmainjector from a Group III metal target. A nitrogen-containing gas isintroduced into the reaction chamber. The III/V ratio of Group III metalsource vapor to nitrogen is adjusted to create a Group III metal-richenvironment within the reaction chamber that is conducive topreferential column growth. The Group III metal source vapor is reactedwith a nitrogen-containing gas to produce a reactant vapor speciescomprising Group III metal and nitrogen. The reactant vapor species isdeposited on the growth surface of the template material to producesingle-crystal M^(III)N columns thereon.

The sputter transport device comprises a sealable or evacuable, pressurecontrolled chamber defining an interior space, a target cathode disposedin the chamber, and a substrate holder disposed in the chamber andspaced at a distance from the target cathode. The target cathode ispreferably bonded to a target cathode holder and negatively biased. Amagnetron assembly is disposed in the chamber proximate to the targetcathode. A negatively-biased, non-thermionic electron/plasma injectorassembly is disposed between the target cathode and the substrateholder. The injector assembly fluidly communicates with a reactive gassource and includes a plurality of hollow cathode-type structures. Eachhollow cathode includes an orifice communicating with the interior spaceof the chamber.

According to one aspect of the present invention, the electron/plasmainjector assembly is adapted for non-thermionically supplying plasma toa reaction chamber. The injector assembly comprises a main body and aplurality of replaceable or interchangeable gas nozzles. The main bodyhas a generally annular orientation with respect to a central axis, andincludes a process gas section and a cooling section. The process gassection defines a process gas chamber and the cooling section defines aheat transfer fluid reservoir. The gas nozzles are removably disposed inthe main body in a radial orientation with respect to the central axisand in heat transferring relation to the heat transfer fluid reservoir.Each gas nozzle provides fluid communication between the process gaschamber and the exterior of the main body.

Methods of the invention entailing the use of the non-thermionicelectron/plasma injector assembly can be utilized to grow a bulk,single-crystal M^(III)N article on the M^(III)N columns. The M^(III)Narticle can be released to provide a free-standing article. Inconjunction with any of the methods of the present invention,microelectronic or optoelectronic devices and/or components can befabricated on the M^(III)N layers or articles, or on any additionallayer grown on the M^(III)N layers or articles.

It is therefore an object of the present invention to provide a methodfor fabricating single-crystal Group III nitride layers or arrays ofcolumns characterized by, among other advantageous properties, lowdefect density and high degree of orientation.

It is another object of the present invention to provide such columnarstructures as strain-relieving buffer or transition layers or seedcrystals for the growth of additional low-defect density Group IIImaterials thereon.

It is yet another object of the present invention to provide a novelsputter material transport method and device capable of ultra-highdeposition and growth rates of low-defect density, strain-relievingGroup III nitride column structures and layers or articles on suchstructures.

Some of the objects of the invention having been stated hereinabove,other objects will become evident as the description proceeds when takenin connection with the accompanying drawings as best describedhereinbelow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevation view of a heterostructure that includes alayer of single-crystal columns produced in accordance with the presentinvention;

FIG. 2 is a perspective view of the heterostructure illustrated in FIG.1 without an intermediate or buffer layer shown;

FIG. 3 is a perspective view of a Group III nitride crystal column grownaccording to the present invention;

FIG. 4 is a perspective view of the heterostructure illustrated in FIG.2, showing a layer grown by columnar epitaxial overgrowth and a bulkarticle grown on the overgrowth layer in accordance with the presentinvention;

FIG. 5 is a side elevation of a free-standing, bulk article producedaccording to the present invention;

FIG. 6A is a side elevation view of the heterostructure illustrated inFIG. 4 with a device fabricated thereon;

FIG. 6B is a side elevation view of the bulk crystal illustrated in FIG.5 with a device fabricated thereon;

FIGS. 7A, 7B, and 7C are scanning electron microscope (SEM) micrographsin top plan view showing columnar structures grown in accordance withthe present invention;

FIGS. 8A, 8B, and 8C are scanning electron microscope (SEM) micrographsin cross-sectional view showing columnar structures grown in accordancewith the present invention;

FIG. 9 is a transmission electron microscope (TEM) micrograph incross-section view showing an upper portion of columnar structures grownin accordance with the present invention;

FIGS. 10A, 10B, and 10C are scanning electron microscope (SEM)micrographs in cross-sectional view showing columnar structures grown ona continuous layer, followed by growth of another continuous layer onthe columnar structures, in accordance with the present invention;

FIGS. 11A and 11B are (0002) ω scans comparing a continuous galliumnitride layer and a columnar gallium nitride layer, respectively,produced according to the invention;

FIGS. 12A and 12B are comparative pole plots of the (1012) planes of acontinuous gallium nitride layer and a columnar gallium nitride layer,respectively, produced according to the invention;

FIG. 13 is a schematic view of a novel sputter transport deviceaccording to one embodiment of the present invention;

FIG. 14A is a top plan view of an electron/plasma injector assemblyprovided according to one embodiment of the present invention;

FIG. 14B is a cut-away vertical cross-sectional view of the injectorassembly illustrated in FIG. 14A taken along line 14B—14B thereof;

FIG. 15 is a schematic view of a novel sputter transport deviceaccording to a further embodiment of the present invention;

FIG. 16 is a perspective view of an electron/plasma injector assemblyaccording to another embodiment of the present invention;

FIG. 17 is a top plan schematic view of the injector assemblyillustrated in FIG. 16;

FIG. 17A is a vertical cross-sectional view of the injector assemblyillustrated in FIG. 17 taken along line 17A—17A thereof;

FIG. 17B is a vertical cross-sectional view of the injector assemblyillustrated in FIG. 17 taken along line 17B—17B thereof;

FIG. 18A is another perspective view of the injector assemblyillustrated in FIG. 16;

FIG. 18B is a top plan view of the injector assembly illustrated in FIG.16;

FIG. 19 is a perspective view of the injector assembly illustrated inFIG. 16 showing the operation thereof and an exemplary electron/plasmainjection pattern;

FIG. 20 is a plot comparing the source performance of a transport deviceprovided according to the present invention and that of a conventionalmagnetron source;

FIG. 21 is a perspective view of a rectangular magnetron source whichcan be employed in combination with the present invention;

FIG. 22 is a schematic view of a novel sputter transport deviceaccording to an additional embodiment of the present invention; and

FIG. 23 is a schematic view of a novel sputter transport deviceaccording to a yet another embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

For purposes of the present disclosure, it will be understood that whena given component such as a layer, region or substrate is referred toherein as being disposed or formed “on” another component, that givencomponent can be directly on the other component or, alternatively,intervening components (for example, one or more buffer or transitionlayers, interlayers, electrodes or contacts) can also be present. Itwill be further understood that the terms “disposed on” and “formed on”are used interchangeably to describe how a given component is positionedor situated in relation to another component. Hence, the terms “disposedon” and “formed on” are not intended to introduce any limitationsrelating to particular methods of material transport, deposition, orfabrication.

The terms “M^(III)N,” “M^(III) nitrides,” and “Group III nitrides” areused herein to describe binary, ternary, and quaternary Group IIInitride based compounds such as aluminum nitride, gallium nitride,indium nitride, aluminum gallium nitride, indium gallium nitride andaluminum indium gallium nitride, and alloys thereof, with or withoutadded dopants or other intentional impurities, as well as all possiblecrystalline structures and morphologies, and any derivatives or modifiedcompositions thereof.

Terms relating to crystallographic orientations, such as Miller indicesand angles in relation to the plane of a layer of material, are intendedherein to cover not only the exact value specified (e.g., (116), 45° andso on) but also any small deviations from such exact value that might beobserved.

As used herein, the term “epitaxy” generally refers to the formation ofa single-crystal film structure on top of a crystalline substrate.Epitaxy can be broadly classified into two categories, namelyhomoepitaxy and heteroepitaxy. In the case of homoepitaxy, the film andthe underlying substrate have the same structural relationships. Reasonsfor extending the substrate through the deposition thereon of anepitaxial film layer, or “epilayer,” of the same composition include theobservations that the epitaxial layer (1) is typically much more free ofdefects as compared to the substrate, (2) is typically purer than thesubstrate, and (3) can be doped independently of the substrate. Therespective lattice parameters of the epilayer and substrate areperfectly matched, with no interfacial bond straining.

In heteroepitaxy, on the other hand, the film and substrate havedifferent compositions. Moreover, the respective lattice parameters are,by definition and to a varying degree, mismatched in the case ofheteroepitaxy. Heteroepitaxy has been accomplished in some processesthat result in a quite small lattice mismatch, such that theheterojunction interfacial structure is similar to a homoepitaxialstructure. Nevertheless, thermal mismatch (i.e., a difference in therespective thermal expansion coefficients between the film andsubstrate) as well as distinctions in the respective chemistries of thefilm and substrate can exist to degrade electronic properties andinterface quality. If the lattice parameters are significantlymismatched, relaxed epitaxy or strained epitaxy results. In the case ofrelaxed epitaxy, misfit dislocation defects form at the interfacebetween the film and the substrate. In the case of strained epitaxy, therespective lattices of the film and the substrate tend to strain inorder to accommodate their differing crystallographies.

As used herein, the term “device” is interpreted to have a meaninginterchangeable with the term “component.”

Referring now to FIGS. 1 and 2, a heterostructure, generally designated10, is illustrated according to the invention. Heterostructure 10comprises a base substrate 12 on which a layer comprising an array ofindividual, single-crystal, low-defect density M^(III)N columns 14 areepitaxially grown. Preferably, base substrate has a diameter of 0.5 inchor greater. Base substrate 12 has a growth surface 12A having acomposition and structure that enables base substrate 12 to serve as atemplate for the epitaxial growth of M^(III)N columns 14 thereon.Alternatively, as shown in FIG. 1, a intermediate buffer or transitionlayer 16 is grown on base substrate 12 so as to provide a suitableepitaxy initiating growth surface 16A for M^(III)N columns 14.

Each column 14 is observed to be distinct from adjacent columns 14, aseach column 14 is separated from other columns 14 over a substantialcolumnar thickness or height t by voids or spaces 18. It can also beseen that columns 14 are highly oriented in the vertical direction andhave substantially perfect flat top surfaces. The areal density ofcolumns 14 over growth surface 12A or 16A is observed to be in the rangeof approximately 25% to approximately 90%. The growth of columns 14 cancontinue such that the thickness of the column layer exceeds that ofintermediate layer 16. The defect density of columns 14 is observed tobe approximately 10⁸ defects per cm² or less, and is lower than that ofbase substrate 12 and/or interlayer 16. The single-crystal, low-defectdensity nature of each column 14, and the discontinuity of the layer ofarray of columns 14, conjoin to provide a strain-relieving,stress-relieving, epitaxial-initiating, columnar growth surface 14A uponwhich M^(III)N layers can be grown with low stress and even lower defectdensity than that of columns 14.

Non-limiting examples of material compositions suitable for use as basesubstrate 12 include sapphire, silicon, silicon carbide, diamond,lithium gallate, lithium aluminate, ScAlMgO₄, zinc oxide, spine,magnesium oxide, gallium arsenide, glass, tungsten, molybdenum, hafnium,hafnium nitride, zirconium, zirconium nitride, carbon,silicon-on-insulator, carbonized silicon-on-insulator, carbonizedsilicon-on-silicon, and gallium nitride. Moreover, the particular basesubstrate selected could be characterized as being a conductivesubstrate, an insulating substrate, a semi-insulating substrate, atwist-bonded substrate, a compliant substrate, or a patterned substrate.

Non-limiting examples of material compositions suitable for use asinterlayer 16 include gallium nitride, aluminum nitride, indium nitride,zinc oxide, silicon carbide, and their alloys. Interlayer 16 could alsobe composed of SiO₂, Si_(x)N_(y), diamond, lithium gallate, lithiumaluminate, zinc oxide, spinel, magnesium oxide, gallium arsenide,tungsten, molydenum, hafnium, hafnium nitride, zirconium, zirconiumnitride, and carbon.

Preferably, base substrate 12 has a thermal coefficient of expansionthat is substantially equal to that of M^(III)N columns 14 in order tominimize thermal mismatch. When interlayer 16 is first deposited ongrowth surface 12A, however, thermal mismatch as between base substrate12 and M^(III)N columns 14 is of less concern.

According to one method of the invention, base substrate 12 and a GroupIII metal target are loaded into a sputter deposition chamber. A highlyenergetic plasma environment is generated in the chamber, using asuitable background gas such as argon. Separate nitrogen-containingsource gas is conducted into the chamber. Alternatively, the gasutilized to generate the plasma could also be used as the reactantsource gas, in which case the background gas provides the nitrogenspecies. The Group III metal target is sputtered to produce a Group IIImetal source vapor. The Group III metal source vapor combines with thenitrogen-containing gas, which is characterized as including one or morespecies such as diatomic nitrogen, atomic nitrogen, nitrogen ions, andpartially ionized nitrogen, as well as nitrogen-containing compoundssuch as ammonia. As a result, reactant vapor species comprisingcomponents of the Group III metal and the nitrogen are produced withinthe reaction chamber, and are deposited on growth surface 12A of basesubstrate 12 or, if applicable, on growth surface 16A of buffer ortransition layer 16.

If the above steps were to be performed in a more conventional manner,the depositing reactant vapor species could be expected to form atwo-dimensional oriented thin film on growth surface 12A (or on growthsurface 16A when first depositing buffer or transition layer 16) and/orthree-dimensional island growth. In accordance with the presentinvention, however, the IN ratio—that is, the ratio of Group III speciesto nitrogen species in the reaction chamber—is controlled duringdeposition of the reactant vapor species. The III/V ratio is maintainedwithin a certain range to create a Group III metal-rich environment inthe reaction chamber that is conducive to and promotes the novelcolumnar growth. In this novel method, the as-deposited reactant vaporspecies has been discovered to grow epitaxially on growth surface 12A ina preferential vertical or three-dimensional orientation to producediscrete, single-crystal M^(III)N columns 14 as depicted in FIGS. 1 and2. The growth of discrete columns 14 thus can be distinguished from theconventional growth of a continuous single crystal or a polycrystallinelayer exhibiting grain boundaries. Importantly, the columnar growth canbe distinguished from the growth of posts or stripes through acombination of conventional masking, etching, lateral growth, andselective area growth techniques. In the present invention, the columnarcrystals are grown in an essentially continuous, one-step process. Themethod of the invention can also be characterized as an in-situ growthprocess, because the columns are formed without having to break thevacuum of the reaction chamber in order to perform extraneous steps suchas masking or otherwise remove the structure from the reaction chamberprior to completion of the growth step.

As appreciated by persons skilled in the art, the III/V ratio is adimensionless quantity that is controlled or adjusted in a manner thatwill depend on the means employed for carrying out the materialtransport process. When, for example, the nitrogen-containing gas isutilized both as the reactant gas and as the background or working gasfor striking and maintaining the plasma or glow discharge in thereaction chamber, the III/V ratio is preferably controlled bycontrolling the voltage applied to the target cathode. When, on theother hand, a non-nitrogen containing background gas such as argon isutilized, the III/V ratio is preferably controlled by controlling theflow of the non-nitrogen containing gas into the reaction chamber. Inthis latter case, control of the III/V ratio during deposition entailsmaintaining a nitrogen concentration in the reaction chamber in therange of approximately 50% to 65% relative to the background gas. Itwill be understood, however, that controlling the nitrogen concentrationadjusts the III/V ratio by altering only the Group V component, and thusassumes that the concentration of the Group III component remain more orless predetermined or fixed. Hence, when depositing material in asputtering regime, the III/V ratio can also be altered by adjusting thevoltage applied to the target cathode. It thus can be seen that theIII/V ratio can be modified to obtain the unique columnar growth byeither adjusting the Group III component, adjusting the Group Vcomponent, or adjusting both the Group III and Group V components.

In order to produce the unique array of columns 14 just described,another important step in the process is the technique by which thecomponents of M^(III)N layer 14 are transported to growth surface 12A ofbase substrate 12. According to the invention, sputtering is performedin favor of other physical vapor deposition techniques, as well as infavor of chemical vapor deposition and other vapor phase techniques.Preferably, the sputtering is accomplished by a novel non-thermionic,plasma-enhanced sputtering technique as described hereinbelow. If aninterlayer 16 is to be formed on growth surface 12A, such interlayer 16can be deposited by any number of techniques, including physical vapordeposition, sputtering, molecular beam epitaxy, atmospheric chemicalvapor deposition, low pressure chemical vapor deposition,plasma-enhanced chemical vapor deposition, metallorganic chemical vapordeposition, sublimiation, evaporation, and hydride vapor phase epitaxy.

Referring now to FIG. 3, a simplified line drawing representing one ofthe single crystal M^(III)N columns 14 shows a hexagonal crystal system.The average thickness or height t of columns 14 produced according tothe invention exceeds 0.5 micron, and ranges from approximately 0.5 to10 microns. The average lateral dimension of columns 14, which can betaken as being either a distance w₁ along an edge of the hexagonalcross-section or a distance w₂ from one vertex to an opposing vertex,ranges from approximately 0.05 micron to 5 microns.

Referring now to FIG. 4, once columns 14 have begun to form during thegrowth process, the III/V ratio can be readjusted to create anenvironment within the reaction chamber that is conducive orpreferential to two-dimensional growth rather than vertically-orientedcolumnar growth. In the example given hereinabove in which the nitrogenconcentration was first adjusted to a range conducive to columnargrowth, the nitrogen concentration can be readjusted to a range betweenapproximately 65% to approximately 100% relative to the argon backgroundgas. In a process termed herein as columnar epitaxial overgrowth, orCEO, the upper regions of columns 14 begin to coalesce and grow as acontinuous M^(III)N layer, which is designated herein as a columnarepitaxial overgrowth or CEO layer 20. CEO layer 20 is characterized as athin film with a preferred thickness of at least approximately 0.5micron or greater.

According to another aspect of the invention, the growth of CEO layer 20is permitted to continue until its thickness is sufficient to ensurethat the bulk crystal has a defect density low enough to be consideredas device-quality. An example of a CEO layer 20 suitable for use as adevice layer has a thickness of at least approximately 0.5 micron orgreater. The structure illustrated in FIG. 4 is then removed from thereaction chamber, and base substrate 12, interlayer 16 (if present),and/or columns 14 are separated or removed from CEO layer 20. Theremoval technique employed can be, for example, polishing,chemomechanical polishing, laser-induced liftoff, cleaving, wet etching,or dry etching of base substrate 12, interlayer 16, and/or columns 14.

According to an additional aspect of the invention, M^(III)N CEO layer20 serves as an excellent seed crystal for either the homoepitaxial orheteroepitaxial growth of a bulk M^(III)N layer 30, as also shown inFIG. 4. In the case of heteroepitaxy, it will be understood that adifferent metal or metal nitride compound source is utilized, which mayrequire that the heterostructure be transferred to a different reactionchamber. Bulk M^(III)N layer 30 can be deposited by any number oftechniques. In addition to the non-thermionic sputter transporttechnique disclosed in detail hereinbelow, bulk M^(III)N layer 30 can begrown by conventional sputtering or other physical vapor depositiontechniques, as well as molecular beam epitaxy, atmospheric chemicalvapor deposition, low pressure chemical vapor deposition,plasma-enhanced chemical vapor deposition, metallorganic chemical vapordeposition, sublimation, evaporation, and hydride vapor phase epitaxy.As diagrammatically represented in FIG. 4 by vertical lines, the defectdensity of the crystal constituting M^(III)N layer 30 is even lower thanthat of CEO layer 20.

Preferably, the growth of bulk M^(III)N layer 30 is permitted tocontinue until its thickness is sufficient to ensure that the bulkcrystal has a defect density low enough to be considered asdevice-quality. In addition, bulk M^(III)N layer 30 can be rotated as itgrows according to conventional methods. The structure illustrated inFIG. 4 is then removed from the reaction chamber, and base substrate 12,interlayer 16 (if present), columns 14, and/or CEO layer 20 areseparated or removed from bulk M^(III)N layer 30. The removal techniqueemployed can be, for example, polishing, chemomechanical polishing,laser-induced liftoff, cleaving, wet etching, or dry etching of basesubstrate 12, interlayer 16, columns 14, and/or CEO layer 20.

During their respective growth steps, columns 14, CEO layer 20, and/orbulk M^(III)N layer 30 can be doped by conducting conventional dopingmethods, such as by introducing dopant-containing gases into thereaction chamber under controlled conditions. Multiple or alternatinglayers of dopants can be added to form electronic devices or componentssuch as, for example, p-n junctions.

Referring to FIG. 5, upon completion of the bulk layer release process,a bulk, free-standing, single-crystal M^(III)N article 40 is produced.In accordance with the invention, article 40 has a diameter d of 0.5inch or greater, a thickness t of 50 microns or greater, and a defectdensity of no greater than 10⁹ defects/cm³. Article 40 can be dopedaccording to known methods. In the production of alloys and compounds,the resulting composition can have greater than 50% Group III metal andnitrogen components.

Bulk crystal article 40 shown in FIG. 5 can be produced in the form of awafer, in which case the thickness will be in the range of 50 microns to1 mm. Multiple wafers can be produced either one at a time or byproviding more than one base substrate 12 in the reaction chamber. Inaddition, the method of the invention enables the production of bulkcrystal article 40 in the form of a boule, in which case the diameter isat least 2 inches and the thickness is from between 1 mm to greater than100 mm. Multiple wafers can be cut from the boule using a wafer saw. Aspart of a further fabrication process, a major surface of the wafer canbe prepared for epitaxial growth according to known methods, such aspolishing, after which an epitaxial layer of suitable composition can bedeposited on the prepared surface.

Referring now to FIGS. 6A and 6B, columns 14, CEO layer 20, M^(III)Nlayer 30, and bulk M^(III)N article 40 produced according to theinvention are device-quality materials that can serve as respectiveplatforms for the fabrication of one or more microelectronic devices,optoelectronic devices, and/or other electronic components 36.Non-limiting examples of devices or components 36 include light-emittingdiodes, detectors, biological or chemical sensors, filters, transistors,rectification circuitry, semiconductor lasers, bond pads, metallizationelements, and interconnects. FIG. 6A specifically illustrates a deviceor component 36 formed on or in M^(III)N layer 30. FIG. 6B specificallyillustrates a device or component 36 formed on or in bulk M^(III)Narticle 40. It will be further understood that an appropriate device orcomponent 36 can be formed on or in CEO layer 20 as well as on one ormore columns 14.

Referring now to FIGS. 7A-10, the results of one series of process runsemploying gallium and nitrogen sources are shown. The columnar growthprocess was performed at an increased III/V ratio (i.e., in agallium-rich environment with a nitrogen concentration of 50%). Theseprocess runs resulted in the columnar growth of single-crystal galliumnitride. As shown in the micrographs of FIGS. 7A, 7B and 7C, flat GaN(0001) surfaces are observed at the tops of randomly distributed,highly-oriented, non-contiguous GaN columns. This effect is most likelydue to an increase in the surface mobility of the gallium species. FIGS.7A-7C show the varying sizes and density of the columns in plane view.FIGS. 8A-8C, show the varying sizes and density of the columns incross-sectional view. The columns are highly oriented in both the basaland prismatic planes. As seen in FIG. 9, cross-sectional TEMmeasurements of the columns reveal that the columns are substantiallydefect-free, thereby indicating high-quality GaN growth.

Examples of columnar epitaxial overgrowth are shown in the SEMcross-sectional images of FIGS. 10A, 10B, and 10C. A GaN columnar regionis observed to have grown on a continuous lower layer, which is followedby the growth of a continuous upper layer. The defect density of theupper layer is lower than that of the lower layer.

The crystal orientation of the columns was examined using X-raydiffraction (XRD) and compared to data obtained for a continuous GaNlayer. Rocking curve measurements of symmetric and asymmetric crystalreflections were examined, and Φ scans and pole figure analysis wereperformed. The rocking curve (ω scan) measurement is indicative of thesubgrain tilt in the crystalline material. As shown in FIGS. 11A and11B, respectively, the FWHM for the 10-micron GaN layer is 249 arcsec,and the FWHM for the GaN column structure is 788 arcsec. As shown inFIGS. 12A and 12B, respectively, pole figure analysis and Φ scans ofboth the GaN layer and the GaN column structure reveal the 6-foldsymmetry of the hexagonal crystal, with no extra peaks from misorientedsubgrains. The similarities of the respective w scan and pole plot ofthe GaN layer and the column structure indicate comparable crystalquality between the two structures and a high degree of crystalorientation in the columns.

In the embodiments of the invention described hereinabove, columnargrowth, columnar eptiaxial overgrowth, and optionally M^(III)N articlelayer growth, are successfully accomplished in part by implementing ahigh-growth rate sputtering technique. The sputtering process isaccomplished by either conventional techniques or, in a preferredprocess, by implementing a novel enhanced sputtering technique describedhereinbelow.

A conventional sputtering technique utilizes a parallel-plate, planardiode configuration in which a cathode and an anode spaced apart fromeach other in a sealable reaction chamber by an electrode gap. Thecathode is driven negative by a power supply. A glow-discharge plasma isgenerated between the two electrodes and confined by a grounded metalvacuum containment wall of the reaction chamber. To “strike” (initiate)the discharge, it is often necessary to supply a spike of highervoltage, or to adjust pressure to a minimum so that the gas will breakdown at the voltage available. The voltage drop across the sheath of theplasma results in high-energy ion bombardment of the cathode by positiveions and sputtering of the cathode. The cathode voltage drop alsosustains the plasma by accelerating secondary electrons emitted from thecathode into the plasma where they initiate a cascade of ionizingcollisions. The diode can be operated under an applied DC voltage or anRF voltage. RF excitation is required when sputtering insulatingtargets.

A mode of plasma-enhanced chemical activation generally known as“reactive sputtering” uses a sputtered source material along with agaseous one. The gas becomes dissociated in the sputtering plasma andreacts to form a compound film. The parallel-plate plasma configurationcan be used to supply vapor for film deposition by sputter-erosion ofthe cathode, which serves as the target material. Often, the plasma ismagnetized using a magnetron assembly. A reactive gas (e.g., N₂) isadded to the sputtering plasma (e.g., argon gas plasma) in order toshift compound-film stoichiometry in sputtering from a compound target,or to deposit a compound film from a metallic target (e.g., Al).Compound deposition by reactive sputtering from a metallic targetgenerally lowers target fabrication costs and increases target purity ascompared to using a compound target, although process control can bemore difficult if film composition is critical.

When employing a planar-diode plasma configuration to cause sputtering,the beam electrons ejected from the cathode must undergo enough ionizingcollisions with the gas to sustain plasma before the beam electronsreach the anode and are removed there. This requirement places a lowerlimit on operating pressure, and can be enhanced through the use of amagnetron assembly. The magnetron assembly typically includes a centralbar magnet and an outer ring magnet or magnets of opposite pole. Themagnetron produces a cross-wise magnetic field over the cathode. Themagnetic field traps the beam electrons in orbits near the cathodesurface. As a result, the path lengths of the beam electrons aresignificantly increased before the electrons finally escape to the anodeby collisional scattering. Because the paths of the electrons becomelonger than the electrode gap, the minimum pressure needed to sustainthe plasma is much lower (typically 0.1 Pa rather than 3 Pa) when usinga magnetron as compared with a planar diode without a magnetron. At alower pressure (e.g., 0.1 Pa), the sputtered particles retain most oftheir kinetic energy upon reaching the substrate, and this energy hasadvantageous effects on the structure of the depositing film. Inaddition, deposition rate is increased due to reduced scattering andredeposition of sputtered particles on the cathode. Moreover, the beamelectrons are utilized more efficiently, with the result that a lowerapplied voltage (e.g., approximately 500 V) is required to sustain aplasma of a given density, and the voltage increases less steeply withpower input as compared to a non-magnetron planar diode configuration.

A typical magnetron has a planar, circular configuration. The targetmaterial of the cathode is a disc, typically 3-10 mm thick, and isbonded (such as by soldering, for good thermal contact) to awater-cooled copper backing plate. The water coolant can be deionized toprevent electrolytic corrosion between the electrically-biased backingplate and a grounded water supply. The cathode is often floated offground with a ceramic insulating ring. The containment wall of thereaction chamber serves as an anode, although grounded shields can beadded to confine the sputtered material. The cross-wise magnetic fieldis established by the magnets. The magnets are connected on the back byan iron “field-return” plate to complete the magnetic circuit and toconfine the magnetic field.

Upon igniting plasma, beam electrons emitted from the cathode areaccelerated into plasma by the electric field of the cathode sheath. Thepresence of the magnetic field causes the beam electrons to curve intoorbits as a result of the Lorentz force, F=F_(E)+F_(B)=q_(e)E+q_(e)v×B.The radius of the orbit (referred to as the gyratron, cyclotron orLarmor radius) depends on the strength of the magnetic field and on theelectron velocity component perpendicular to the magnetic field. Inorder for the magnetic field to have an effect on the beam electrons,the pressure must be low enough (typically less than a few Pa) that theelectron mean free path is not significantly less than the orbit radius.If this condition is met, the beam electrons are said to be “magnetized”although the ions are not magnetized. The magnetron can operate as asputtering source at much higher pressures, but in such cases gasscattering dominates the behavior of the beam electrons instead of themagnetic field itself.

Under lower pressure conditions, the beam electrons emitted from thetarget surface of the cathode or created by ionization in the sheathfield are accelerated vertically by the electric field andsimultaneously forced sideways by the magnetic field. The beam electronseventually reverse direction and return toward the target. As the beamelectrons are thus directed toward the target, they decelerate in theelectric field until their direction is again reversed, and the cyclerepeats. The net motion or path of these electrons is a circular driftpath around the circle of the target. This drift path is in thedirection of the E×B vector product. The magnetron is ordinarilydesigned such that the E×B drift path closes on itself so that the beamelectrons do not pile up or accumulate at some location.

Preferably, the plasma generated in the reaction chamber is enhanced bytaking advantage of the “hollow cathode” effect, a phenomenon whichgenerally involves utilizing geometric means to trap secondary electronsemitted from an ion-bombarded target cathode. When a hollow-cathode-typestructure is driven to a very high discharge current, its cathodesurfaces heat to a temperature sufficient to cause thermionic emissionof electrons, and the local plasma glow discharge will enter the arcmode. A hollow cathode, typically constructed of a refractory materialand provided with a local gas supply, can be a useful source ofmoderately energetic electrons for plasmas. The hollow cathode isprovided in the form of a tube having a tantalum tip. A gas source isconnected to one end of the hollow cathode, and a small aperture ororifice is provided at the tip. The aperture restricts the gas flow andresults in a large pressure differential across tip. The inner pressureof the hollow cathode is typically in the range of several hundredmTorr. Electrons are emitted by biasing the hollow cathode negativelywith respect to the local plasma potential (which is usually the groundpotential). A hollow cathode having a diameter of only a few millimeterscan be employed to produce an electron current of several to tenamperes. An external heater or a short-term, high-voltage spike istypically used to heat the hollow cathode to the temperature requiredfor emission.

The hollow cathode is situated in the fringe region of the magneticfield of the magnetron to supply additional electrons to the magnetrondischarge. The hollow cathode serves to decouple the current-voltagerelation of the diode plasma and allow operation of the plasma at wideranges of voltage and current, as well as to lower the operatingpressure in chamber. The hollow cathode can operate at 0.1 mTorr, whichis below the range of the more conventional magnetron/diode arrangementdescribed hereinabove. If conventional magnetron/diode arrangements wereto operate at these lower pressures, there would be not be enough gasatoms for efficient ionization by the secondary electrons. Theadditional supply of electrons from the hollow cathode, however, removesthis limitation and allows operation at approximately 0.1 mTorr formagnetron arrangements, and approximately 0.5 mTorr for RF-diodearrangements. Such pressures are well into the long mean free path mode,and sputtered atoms or ions move in straight, line-of-sight trajectorieswithout gas scattering.

While hollow cathode enhanced sputtering devices provide advantages overother sputter deposition techniques, there are still drawbacks withregard to their use, owing to the fact that they are thermionic emittingelectron devices. For instance, contamination is still observed to be aproblem, particularly since the hollow cathode tip material tends toevaporate and mix with the growing deposition material. Another problemrelates to the intense heat produced by thermionic emission, which candamage the growing material. Therefore, in accordance with a preferredembodiment of the present invention, a novel sputter transport device isprovided that is characterized by the use of a non-thermionicelectron/plasma injector assembly.

Referring now to FIG. 13, a non-thermionic sputter transport device,generally designated 100, is illustrated. Key operating components oftransport device 100 are contained within a grounded, sealablesputter-transport chamber 102. As will be appreciated by persons skilledin the art, a pumping system (not shown) is provided to control thepressure (vacuum or otherwise) within chamber 102. Supply systems (notshown) are also provided for delivering a background gas (e.g., argon),and a reactive gas (e.g., nitrogen) in the case of reactive sputtering,into chamber 102. In some applications of the present invention, thereactive gas may also serve as the background gas.

A cathode 104 constructed from a metallic, dielectric, or compoundtarget material is bonded to a target holder 106 to establish thermalcontact therebetween. Target cathode 104 may be provided in the form ofa circular disk or a rectilinear plate, or may have some other shape.Target holder 106 is preferably constructed of copper or otherrelatively inexpensive material that offers acceptable levels of boththermal and electrical conductivity. A heat exchanger system (not shown)is provided to circulate a heat transfer medium such as water throughtarget holder 106 to keep target holder 106 (and thus target cathode104) cool. A magnetron assembly 110 includes a set of oppositely-poledmagnets 112 and 116 connected by a magnetic field return plate 118. Thearrangement of magnets 112 and 116 preferably constitutes a centralmagnetic bar 112 surrounded by an outer magnetic annulus 116, althoughother arrangements and shapes could be provided. Magnets 112 and 116 arepreferably located on the side of target holder 106 opposite to targetcathode 104. A negative bias voltage is applied to target holder 106 byconnecting target holder 106 in series with a voltage source 120.

A substrate holder 130, which serves as the primary anode, is disposedin chamber 102 in parallel with and spaced at a distance from targetcathode 104. Preferably the spacing is in the range of approximately 2cm to 20 cm. Substrate holder 130 can be constructed from any materialthat is either electrically conductive or isolated, and can be providedas either a cooling structure or a heating structure. It is preferablethat transport device 100 be oriented such that target cathode 104 isphysically situated opposite to substrate holder 130, but can be eithervertically above or below substrate holder 130. A substrate 132 isdisposed on substrate holder 130. Depending on the specific applicationof transport device 100, substrate 132 can be either initially providedin bulk form on which a thin-film is to be deposited, or it representsthe growing bulk material grown through use of transport device 100.

As will be appreciated by persons skilled in the art, substrate holder130 or an associated transfer arm (not shown) can be used to transportsubstrate holder 130 and, if applicable, an initially-provided substratematerial into and out from chamber 102. In addition, a load lock orsimilar component (not shown) can be provided to serve as an interfacebetween chamber 102 and the ambient environment to assist in maintainingreduced pressure in chamber 102 when substrate holder 130 and/or aninitially-provided substrate material is loaded and thereafter removedfrom chamber 102. Other known processing components can used asappropriate to assist in implementing the methods of the inventioninvolving the use of transport device 100, including an electroniccontrol system, a power supply system, a pressure monitoring system, amass flow control system, a temperature monitoring system, and a systemfor automated tracking and transport of workpieces.

As one key aspect of the present invention, an injector assemblygenerally designated 150 is disposed in chamber 102 proximate to targetcathode 104, and is separately, negatively biased through its serialconnection with a voltage source 152. Hence, injector assembly 150serves as a cathode apart from and additional to target cathode 104,such that transport device 100 can be characterized as being a triodesputtering source.

Referring to FIGS. 14A and 14B, injector assembly 150 includes aplurality of injectors 152 serving essentially as individual hollowcathodes. Each injector 152 terminates in an inlet orifice 152Acommunicating with the interior of chamber 102 in the region proximateto the surface of target cathode 104. In the present embodiment,injector assembly 150 takes the form of an injector ring such that eachinlet orifice 152A faces radially inwardly with respect to chamber 102,although individual injectors 152 can be arranged in a linear or othersuitable configuration.

In operation, electrons in the form of supplemental or auxiliary plasmabeams are non-thermionically emitted from injectors 152 as a result ofthe increase in electric field strength at these points, such that theelectrons are subsequently injected and coupled into the gradient of themagnetic field (represented by virtual field lines B) established bymagnetron source 110 to generate an intense plasma. Injector assembly150 may thus be characterized as a cool, non-thermionic electron/plasmasource which injects an approximately equal number of ions and electronsinto the region illustrated in FIG. 13 proximate to target cathode 104,thereby creating a higher probability of ionization of the targetmaterial. An increase in magnetron current is observed due to the addedelectrons from injector assembly 150. This effect can be seen as asignificant increase in the plasma brightness, as well as a significantincrease in the sputter deposition rate. The intense plasma created inthe proximity of the surface of target cathode 104 results in thesignificant increase in deposition rate by more than ten times overconventional techniques. Injector assembly 150 also serves toelectrostatically confine the plasma to form a broad plasma beam 160directed toward substrate 132. Due to the bulk mass and/or coolingdesign of injector assembly 150, its temperature remains low andaccordingly no thermionic emission, evaporation or contamination takesplace during deposition.

Transport device 100 can be operated in either continuous DC, pulsed DC,AC or RF mode, which enables transport device 100 to reactively sputtera wide range of both conductive and insulating materials at very highrates. Due to the high percentage of gas ionization, the material oftarget cathode 104 is sputtered at ultra-high rates sufficient toprevent a detrimental insulating layer from forming on the targetsurface. In addition, due to the very high ion energies associated withthe process according to the present invention, large amounts ofmaterial can be sputtered. Device 100 has been proven to operatesuccessfully in 100% reactive gas environments, therefore demonstratingthe stability of the device under very reactive conditions.

As described above, a negative bias is applied to target holder 106,which generates a magnetron sputtering discharge, and a separatenegative bias is applied to injector assembly 150. This generates a veryintense plasma, with beamlets of plasma emitting from each injector 152of injector assembly 150. The added plasma density and ionizationpercentage in the region of the target cathode 104 increase the amountof target bombardment, thereby causing increased sputter rates. Due tothe increased utilization of sputtering gas, the background processingpressure can be lowered from, for example, approximately 5 mTorr toapproximately 0.1 mTorr, which can improve the microstructuralproperties of materials being formed. This pressure decrease increasesthe mean free path of molecules, enabling the creation of plasma beam160 between target cathode 104 and substrate holder 130 (i.e., theanode) which is characterized by very high ionization efficiency andachievement of ultra-high sputter transport rates.

Referring to FIG. 15, a sputter transport device, generally designated200, is illustrated according to another embodiment of the presentinvention. In this particular embodiment, a biased containment shield202, constructed from aluminum or other conductive material, is disposedin chamber 102 between target cathode 104 and substrate holder 130 andis surrounded by a containment magnet or magnets 204. A high voltageapplied to containment shield 202 from a voltage source 206 acts tofocus the sputtered material and plasma beam 160 onto the growingsubstrate 132, thereby increasing the transport efficiency of thesputtered material (such as aluminum nitride) to substrate 132. Ions andelectrons become trapped within the containment region under theinfluence of the electric and magnetic fields and subsequently depositon substrate 132.

Under some circumstances, the user of transport device 100 or 200 mightfind that the heating of injector assembly 150 causes low-melting-pointmetals to melt. This problem can be overcome by cooling injectorassembly 150 with a copper cooling ring 220, which is also illustratedin FIG. 15.

Referring to FIGS. 16-19, a preferred embodiment of a fluid-cooled,ring-shaped injector assembly generally designated 300 is illustrated.Injector assembly 300 includes a main body 302 and an outer collar 304removably secured by clamping screws 306. Main body 302 includes aprocess gas section 302A and a cooling section 302B. As best shown inFIGS. 17A and 17B, process gas section 302A and outer collar 304together define a process gas chamber 308. Individual injectors forsupplying electrons and cool plasma, indicated by the reference numeral310, are defined by interchangeable gas nozzles 312 fluidlycommunicating with process gas chamber 308 at one end and withsputter-transport chamber 102 at the other end. Gas nozzles 312 may ormay not be constructed from the same material as target cathode 104and/or containment shield 202. Cooling section 302B of main body 302defines a cooling reservoir 314 adapted to circulate a heat transferfluid such as water in close proximity to each gas nozzle 312. The heattransfer fluid is circulated through cooling reservoir 314 by means of aheat transfer fluid inlet conduit 316 and outlet conduit 318. Processgas such as diatomic nitrogen or argon is supplied to injector assembly300 by means of a process gas conduit system 320 that communicates withone or more process gas inlets 322 on main body 302. FIG. 19 illustratesone example of an emission pattern of plasma/electrons 310 obtainable byinjector assembly 300. The pattern as well as the gas nozzle pressurecan be altered by blocking one or more of individual gas nozzles 312.

Traditionally, sputter-deposited films have been plagued with lowreactive sputter rates, excessive stress, and poor crystalline growth.Due to the non-contaminating nature of transport device 100 or 200,however, the hollow cathode effect can be advantageously utilized toproduce both single-crystal and highly-oriented polycrystalline,bulk-form substrates, such as those described hereinabove, at lowerpressures, ultra-high deposition rates, and with minimal materialstress. Transport device 100 or 200 is also capable of growing epitaxiallayers on substrates. Examples of deposited materials include binary,tertiary, and quaternary Group III nitride based compounds such asaluminum nitride, gallium nitride, indium nitride, aluminum galliumnitride, indium gallium nitride and aluminum indium gallium nitride, andalloys thereof. Suitable dopants can be added during the growth process.Both single-crystal and polycrystalline morphologies are obtainable. Inone specific example, transport device 100 or 200 is capable of growingaluminum nitride purer than that made by powder processing methods andfaster than CVD methods. Moreover, because transport device 100 or 200exhibits a very high degree of sputter particle ionization, transportdevice 100 or 200 produces a plasma beam environment that facilitatesthe synthesis of nitride based materials. The material grown bytransport device 100 or 200 exhibits the bulk properties of nitrides dueto the resulting high crystallinity and purity. In particular, bulkaluminum nitride produced from transport device 100 or 200 has a high IRand UV transmittance, a high thermal conductivity, and a high degree ofc-axis orientation.

In addition to growing the materials described hereinabove, transportdevice 100 or 200 can be utilized to grow a variety of ceramic thinfilms such as aluminum oxide and zinc oxide, or to deposit copper orother metallic interconnects onto patterned electronic devices. The hightransport rate also enables the high-throughput coating of objects.

FIG. 20 demonstrates the dramatic improvement in deposition rate byplotting plasma current as a function of applied source voltage withtransport device 100 operating under a 0.7A electron enhancement (i.e.,with the inventive injector ring installed and supplying current fromhollow cathode-type structures), as compared to a typical magnetronsputtering device without any electron enhancement.

Conventional planar magnetron designs suffer from poor target-materialutilization because of a trenched erosion pattern that tends to form onthe surface of the target material in the vicinity of the E×B drift pathof the beam electrons. The radial narrowness of this trench results fromradial compression of the plasma, which is in turn caused by thewell-known “magnetic-mirror” effect. The electrons of the plasma areforced away from both small and large magnetron radii at the sites wherethe magnetic field converges toward the magnetic pole pieces. Theelectrons are compressed by these mirrors toward an intermediate radiuswhere the magnetic field is uniform. Both the plasma and the ionbombardment are most intense in the region of magnetic field uniformity.The magnetic-mirror effect can be reduced somewhat by designing aflatter magnetic field or by mechanically scanning the magnets back andforth during sputtering. The non-uniformity of film thickness resultingfrom plasma compression can be avoided by moving the substrates aroundduring deposition. One simpler, geometric approach to improvinguniformity is illustrated in FIG. 21, wherein a rectangular magnetrongenerally designated 410 is utilized. With the rectangular geometry, themany of magnetic field lines B are situated along linear directions, andthe beam electrons follow an oblong or “racetrack” E×B drift path attarget cathode 104. The rectangular magnetron shape can be employed inconnection with the present invention if non-uniformity becomesproblematic.

Localization of the plasma over target cathode 104 by the transversemagnetic field of magnetron assembly 110 results in a much lower plasmadensity over the substrate 132 than in the case of the non-magnetronplanar diode, and ion bombardment flux to substrate 132 is reducedaccordingly. This is desirable when the neutral sputtered particlesalone carry sufficient kinetic energy to optimize film structure, orwhen it is important that the substrate heating that results from ionbombardment be kept to a minimum. In other cases, however, it might bedesirable to further increase film bombardment while retaining the lowoperating pressure of the transport device 100 or 200. One method forincreasing ion bombardment of the growing film is to “unbalance” themagnets of magnetron assembly 110, such as by downsizing central magnet112 such that the central magnet 112 cannot pull in all the field linesemanating from outer magnets 116. Hence, in the unbalancedconfiguration, the magnetic field lines that are not pulled into centralmagnet 112 will curve away toward substrate holder 130. Becauseelectrons traveling parallel to a magnetic field are not influenced bythe magnetic field, they can escape along these wayward field lines andtravel toward substrate 132. The escaping electrons pull positive ionsalong with them by ambipolar diffusion and hence increaseion-bombardment flux to substrate 132. In addition, the bombardmentenergy can be increased by negatively biasing substrate 132.

Another way to increase ion-bombardment flux to the growing film is toprovide an RF-powered coil to ionize the mostly neutralsputtered-particle flux during transport to substrate 132. The coiloperates by coupling energy inductively into a secondary plasmadownstream of the magnetron plasma.

Referring now to FIG. 22, a sputter transport device, generallydesignated 600, is illustrated according to an additional embodiment ofthe present invention. Many of the components of sputter transportdevice are similar to those of sputter transport device 100 shown inFIG. 13. In particular injector assembly 150 as described above isutilized to enhance the material transport process. A primary differenceis that a liquid target 604 such as liquid-phase aluminum or gallium isprovided as a source species. The target holder in this embodiment isprovided in the form of a cup 606 to contain the liquid target material.Preferably, this target holder should be constructed from a materialsuitable for withstanding the heat involved and which will notcontaminate the target material. Candidate materials for target holder606 include molybdenum and stainless steel. In one embodiment, a 6″diameter molybdenum liquid gallium or aluminum target holder 606 isemployed to prevent reaction of the holder with a high purity (99.9999%)liquid gallium or aluminum source 604. In order to obtain a flat uniformliquid surface of the gallium or aluminum, sufficient wetting of thegallium or aluminum to the molybdenum holder 606 must occur. To thisend, grooves can be cut into the bottom of target holder 606 to increaseits surface area and thereby increase its wettability. In addition, abreathing hole connecting the grooves can be provided to eliminate anygas trapped under the liquid gallium or aluminum.

Referring now to FIG. 23, a sputter transport device, generallydesignated 700, is illustrated according to another embodiment of thepresent invention. Sputter transport device 700 is equipped with abiased containment shield 202 and containment magnets 204, similar tothose described in reference to FIG. 15. A high voltage applied tocontainment shield 202 will focus the sputtered material onto growingsubstrate or film 132, thereby increasing the transport efficiency of Gaor Al to substrate or film 132.

Sputter transport devices 600 and 700 operate as described above.Gallium (or aluminum) particles sputtered from the cathode react withatomic nitrogen in the cathode magnetic fields. The gallium nitride (oraluminum nitride) particles travel through the containment magneticfield to the substrate. The quality of growth material is determined bythe nucleation and growth at the substrate surface.

EXAMPLE 1

An example of a method for manufacturing a GaN single crystal columnarlayer on a sapphire substrate by sputtering of gallium in a nitrogenenvironment will now be described. Raw materials employed in this methodinclude 99.9999% pure gallium and nitrogen-containing gases such asnitrogen or ammonia. The gallium target used to provide the galliumsource vapor is loaded on a water-cooled magnetron assembly disposed ina vacuum chamber. The nitrogen-containing gas used to provide thenitrogen source vapor is introduced into the vacuum chamber using massflow controllers.

A sapphire wafer is cleaned and placed in a wafer platter. The waferplatter is loaded into the vacuum chamber and placed in contact with asubstrate heater assembly. The vacuum chamber is then, pumped down to10⁻² Torr with a mechanical vacuum pump. A diffusion pump is used toreduce the chamber pressure to 10⁻⁷ Torr. The sample is then heated to atemperature of 1000° C. in 1 hour. The chamber is baked out to apressure of 10⁻⁶ Torr. Nitrogen and argon gas are then introduced intothe vacuum chamber. The total chamber pressure is 10 mTorr, with anargon partial pressure of 5.5 mTorr and a nitrogen partial pressure of6.5 mTorr. The plasma is ignited and set to a power of 500W. The systemis held in this configuration for 1 hour. The plasma is then turned offand the heater is ramped to 25° C. in 5 hours. During these stages, asingle-crystal GaN columnar structure is formed on the sapphire wafer asrepresented by, for example, columnar array 14 in FIG. 1 (butdisregarding the illustrated buffer or transition layer 16). The gasflow is stopped after the crystal has cooled to room temperature. Thesapphire wafer with the resulting GaN crystal columnar layer epitaxiallygrown thereon is then removed from the chamber. The GaN columnar layeron the sapphire is 2 μm thick.

The resulting GaN columnar layer can then be used as a buffer ortransition layer on which various devices, components, and/or additionallayers can then be formed.

EXAMPLE 2

An example of a method for manufacturing a GaN single crystal columnarlayer on a GaN buffer or transition layer on sapphire by sputtering ofgallium in a nitrogen environment will now be described. Raw materialsemployed in this method include 99.9999% pure gallium andnitrogen-containing gases such as nitrogen or ammonia. The galliumtarget used to provide the gallium source vapor is loaded on awater-cooled magnetron assembly disposed in a vacuum chamber. Thenitrogen-containing gas used to provide the nitrogen source vapor isintroduced into the vacuum chamber using mass flow controllers.

A sapphire wafer is cleaned and placed in a wafer platter. The waferplatter is loaded into the vacuum chamber and placed in contact with asubstrate heater assembly. The vacuum chamber is then pumped down to10⁻² Torr with a mechanical vacuum pump. A diffusion pump is used toreduce the chamber pressure to 10⁻⁷ Torr. The sample is then heated to atemperature of 1000° C. in 1 hour. The chamber is baked out to apressure of 10⁻⁶ Torr. Nitrogen and argon gas are then introduced intothe vacuum chamber. The total chamber pressure is 10 mTorr, with anargon partial pressure of 2.5 mTorr and a nitrogen partial pressure of7.5 mTorr. The plasma is turned on at 500W for 1 minute. The temperatureis then held for 1 minute. At this point, a GaN buffer or transitionlayer is formed on the sapphire wafer as represented by, for example,intermediate layer 16 in FIG. 1. The partial pressure of the argon isthen set to 5.5 mTorr and the partial pressure of the nitrogen is set to6.5 mTorr. The plasma is ignited again and set to a power of 500W. Thesystem is held in this configuration for 1 hour. The plasma is thenturned off and the heater is ramped to 25° C. in 5 hours. During thesestages, a single-crystal GaN columnar structure is formed on the bufferor transition layer as represented by, for example, columnar array 14 inFIG. 1. The gas flow is stopped after the crystal has cooled to roomtemperature. The sapphire wafer with the resulting GaN crystal columnarlayer epitaxially grown thereon is then removed from the chamber. TheGaN columnar layer on the sapphire is 2 μm thick.

The resulting GaN columnar layer can then be used as a buffer ortransition layer on which various devices, components, and/or additionallayers can then be formed.

EXAMPLE 3

An example of a method for manufacturing a GaN single crystal layer on aGaN columnar layer by sputtering of gallium in a nitrogen environmentwill now be described. Raw materials employed in this method include99.9999% pure gallium and nitrogen-containing gases such as nitrogen orammonia. The gallium target used to provide the gallium source vapor isloaded on a water-cooled magnetron assembly disposed in a vacuumchamber. The nitrogen-containing gas used to provide the nitrogen sourcevapor is introduced into the vacuum chamber using mass flow controllers.

A sapphire wafer is cleaned and placed in a wafer platter. The waferplatter is loaded into the vacuum chamber and placed in contact with asubstrate heater assembly. The vacuum chamber is then pumped down to10⁻² Torr with a mechanical vacuum pump. A diffusion pump is used toreduce the chamber pressure to 10⁻⁷ Torr. The sample is then heated to atemperature of 1000° C. in 1 hour. The chamber is baked out to apressure of 10⁻⁶ Torr. Nitrogen and argon gas are then introduced intothe vacuum chamber. The total chamber pressure is 10 mTorr, with anargon partial pressure of 2.5 mTorr and a nitrogen partial pressure of7.5 mTorr. The plasma is turned on at 500W for 1 minute. The temperatureis then held for 1 minute. The partial pressure of the argon is then setto 5.5 mTorr and the partial pressure of the nitrogen is set to 6.5mTorr. The plasma is turned on at 500W for 1 hour. The total chamberpressure is then set back to 10 mTorr, with an argon partial pressure of2.5 mTorr and a nitrogen partial pressure of 7.5 mTorr. The magnetronplasma is ignited and set to a power of 5 kW. The system is held in thisconfiguration for 12 hours. The plasma is then turned off and the heateris ramped to 25° C. in 5 hours. The gas flow is stopped after thecrystal has cooled to room temperature. The sapphire wafer with theresulting GaN crystal columnar layer epitaxially grown thereon is thenremoved from the chamber. The GaN layer on the columnar layer is 300 μmthick and 2 inches in diameter.

The GaN layer can then be released from the sapphire template andprepared for use as a substrate. The sapphire template is removed fromthe GaN layer using a known removal technique such as, for example, byusing a mechanical lapping machine. The resulting GaN wafer has athickness of approximately 200 μm and a diameter of approximately of 2inches as represented by, for example, article 40 in FIG. 5. The GaNwafer is then chemically or mechanically polished by known techniques.The polishing step is followed by a dry etching procedure to produce asurface on the GaN wafer receptive to a thin film of GaN. An epitaxiallayer of GaN is then deposited on the prepared surface of the GaN waferto a typical thickness of approximately 1 to 2 microns by an appropriateprocess such as, for example, sputtering, MBE, MOCVD, or HVPE. Variousdevices, components, and/or additional layers can then be formed on theprepared GaN substrate.

EXAMPLE 4

An example of a method for manufacturing a GaN single crystal in bouleform on a sapphire substrate by sputtering of gallium in a nitrogenenvironment will now be described. Raw materials employed in this methodinclude 99.9999% pure gallium and nitrogen-containing gases such asnitrogen or ammonia. The gallium target used to provide the galliumsource vapor is loaded on a water-cooled magnetron assembly disposed ina vacuum chamber. The nitrogen-containing gas used to provide thenitrogen source vapor is introduced into the vacuum chamber using massflow controllers.

A sapphire wafer is cleaned and placed in a wafer platter. The waferplatter is loaded into the vacuum chamber and placed in contact with asubstrate heater assembly. The vacuum chamber is then pumped down to10⁻² Torr with a mechanical vacuum pump. A diffusion pump is used toreduce the chamber pressure to 10⁻⁷ Torr. The sample is then heated to atemperature of 1000° C. in 1 hour. The chamber is baked out to apressure of 10⁻⁶ Torr. Argon gas is introduced into the vacuum chamberthrough the non-thermionic electron/plasma injector assembly describedhereinabove. Nitrogen gas is introduced into the vacuum chamber near thesapphire wafer substrate. The total chamber pressure is 10 mTorr, withan argon partial pressure of 2.5 mTorr and a nitrogen partial pressureof 7.5 mTorr. The plasma is turned on at 500W for 1 minute. Thetemperature is then held for 1 minute. The partial pressure of the argonis then set to 5.5 mTorr and the partial pressure of the nitrogen is setto 6.5 mTorr. The plasma is turned on at 500W for 1 hour. The totalchamber pressure is then set back to 10 mTorr, with an argon partialpressure of 2.5 mTorr and a nitrogen partial pressure of 7.5 mTorr. Avoltage of 100V is applied to the containment shield describedhereinabove, and the containment magnets (also described hereinabove)are turned on. The magnetron plasma is ignited and set to a power of 10kW. The plasma supplied by the injector assembly is ignited and set to apower of 5 kW. The system is held in this configuration for 50 hours.The plasma is then turned off and the heater is ramped to 25° C. in 5hours. The gas flow is stopped after the crystal has cooled to roomtemperature. The sapphire wafer with the resulting GaN crystal columnarlayer epitaxially grown thereon is then removed from the chamber. TheGaN layer on the columnar layer is 30 mm thick and 2 inches in diameter.

One or more device-ready substrates can then be prepared from the GaNboule. The GaN boule is cut using a known technique such as, forexample, by using an inside diameter wafer saw, thereby producing a GaNwafer. The wafer has a thickness of approximately 500 μm and a diameterof approximately 2 inches. The GaN wafer is then chemically ormechanically polished by known techniques. The polishing step isfollowed by a dry etching procedure to produce a surface on the GaNwafer receptive to a thin film of GaN. An epitaxial layer of GaN is thendeposited on the prepared surface of the GaN wafer to a typicalthickness of approximately 1 to 2 microns by an appropriate process suchas, for example, sputtering, MBE, MOCVD, or HVPE. Various devices,components, and/or additional layers can then be formed on the preparedGaN substrate.

It will be understood that various details of the invention may bechanged without departing from the scope of the invention. Furthermore,the foregoing description is for the purpose of illustration only, andnot for the purpose of limitation—the invention being defined by theclaims.

What is claimed is:
 1. A method for producing single-crystal M^(III)Ncolumns comprising the steps of: (a) providing a template materialhaving an epitaxial-initiating growth surface; (b) sputtering a GroupIII metal target in a reaction chamber to produce a Group III metalsource vapor; (c) introducing a nitrogen-containing gas into thereaction chamber; (d) adjusting the III/V ratio of Group III metalsource vapor to nitrogen to create a Group III metal-rich environmentwithin the reaction chamber conducive to preferential column growth; (e)combining the Group III metal source vapor with the nitrogen-containinggas to produce a reactant vapor species comprising Group III metal andnitrogen; and (f) depositing the reactant vapor species on the growthsurface to produce single-crystal M^(III)N columns thereon.
 2. Themethod according to claim 1 wherein the template material comprises acomponent selected from the group consisting of sapphire, silicon,silicon carbide, diamond, lithium gallate, lithium aluminate, ScAlMgO₄,zinc oxide, spinel, magnesium oxide, gallium arsenide, glass, tungsten,molybdenum, hafnium, hafnium nitride, zirconium, zirconium nitride,carbon, silicon-on-insulator, carbonized silicon-on-insulator,carbonized silicon-on-silicon, and gallium nitride.
 3. The methodaccording to claim 1 wherein the template material is selected from thegroup consisting of conductive substrates, insulating substrates,semi-insulating substrates, twist-bonded substrates, compliantsubstrates, or patterned substrates.
 4. The method according to claim 1wherein the template material has a thermal coefficient of expansionsubstantially equal to the M^(III)N columns.
 5. The method according toclaim 1 wherein the template material has a diameter of approximately0.5 inch or greater.
 6. The method according to claim 1 wherein theGroup III metal target comprises a component selected from the groupconsisting of gallium, indium, aluminum, and binary, ternary, andquaternary alloys and compounds thereof.
 7. The method according toclaim 1 wherein the nitrogen-containing gas includes species selectedfrom the group consisting of diatomic nitrogen, atomic nitrogen,nitrogen ions, partially ionized nitrogen, ammonia, nitrogen-containingcompounds, and combinations thereof.
 8. The method according to claim 1wherein the step of adjusting the III/V ratio comprises adjusting thenitrogen concentration in the reaction chamber.
 9. The method accordingto claim 1 wherein the step of adjusting the III/V ratio comprisesadjusting the voltage applied to the metal target.
 10. The methodaccording to claim 1 comprising the step of introducing a background gasinto the reaction chamber, wherein the III/V ratio is adjusted byadjusting the concentration of the background gas.
 11. The methodaccording to claim 1 wherein the growth temperature is set within arange of approximately 400° C. to approximately 1200° C.
 12. The methodaccording to claim 11 wherein the growth temperature is set within arange of approximately 600° C. to approximately 1000° C.
 13. The methodaccording to claim 1 wherein the reactant vapor species is depositeddirectly on the template material.
 14. The method according to claim 1comprising the step of depositing an intermediate layer on the templatematerial prior to depositing the reactant vapor species.
 15. The methodaccording to claim 14 wherein the intermediate layer comprises amaterial selected from the group consisting of GaN, AlN, InN, ZnO, SiC,Si, and alloys thereof.
 16. The method according to claim 14 wherein theintermediate layer comprises SiO₂, Si_(x)N_(y), diamond, lithiumgallate, lithium aluminate, zinc oxide, spinel, magnesium oxide, galliumarsenide, tungsten, molybdenum, hafnium, hafnium nitride, zirconium,zirconium nitride, and carbon.
 17. The method according to claim 14wherein the intermediate layer is deposited by a technique selected fromthe group consisting of physical vapor deposition, sputtering,evaporation, sublimation, molecular beam epitaxy, atmospheric chemicalvapor deposition, low pressure chemical vapor deposition,plasma-enhanced chemical vapor deposition, metallorganic chemical vapordeposition, and hydride vapor phase epitaxy.
 18. The method according toclaim 14 wherein the M^(III)N columns have an average thickness greaterthan the intermediate layer.
 19. The method according to claim 14wherein the M^(III)N columns have a defect density lower than theintermediate layer and the template material.
 20. The method accordingto claim 1 comprising the step of doping the M^(III)N columns.
 21. Themethod according to claim 20 wherein more than one type of dopant isadded to the M^(III)N columns.
 22. The method according to claim 1wherein the M^(III)N columns are formed at a growth rate ofapproximately 1 micron/hour or greater.
 23. The method according toclaim 1 wherein the M^(III)N columns are provided in a form selectedfrom the group consisting of intrinsic M^(III)N, doped M^(III)N, andM^(III)N alloys and compounds containing greater than 50% M^(III) and N.24. The method according to claim 1 wherein the M^(III)N columns have anepitaxial relationship with the template material.
 25. The methodaccording to claim 1 wherein the M^(III)N columns have a density ofapproximately 25% to approximately 90% on the template material.
 26. Themethod according to claim 1 wherein the M^(III)N columns have an averagelateral dimension ranging from approximately 0.05 micron toapproximately 5 microns.
 27. The method according to claim 1 wherein theM^(III)N columns have an average height of approximately 0.5 micron orgreater.
 28. The method according to claim 1 wherein the M^(III)Ncolumns have a defect density of approximately 10⁸ defects per cm² orless.
 29. The method according to claim 28 wherein the M^(III)N columnshave a defect density of approximately 10⁴ defects per cm² or less. 30.The method according to claim 1 wherein the M^(III)N columns have adefect density lower than the template material.
 31. The methodaccording to claim 1 comprising the steps of readjusting the III/V ratioto create an environment within the reaction chamber conducive tocolumnar epitaxial overgrowth, and continuing to deposit the reactantspecies on the growing M^(III)N columns, whereby uppermost regions ofthe M^(III)N columns coalesce and grow to form a substantiallycontinuous, single-crystal M^(III)N layer.
 32. The method according toclaim 31 wherein the step of readjusting the III/V ratio comprisesreadjusting the nitrogen concentration in the reaction chamber.
 33. Themethod according to claim 31 wherein the M^(III)N layer has a defectdensity equal to or lower than the M^(III)N columns.
 34. The methodaccording to claim 31 wherein the M^(III)N layer has a defect density ofapproximately 10⁸ defects per cm² or less.
 35. The method according toclaim 34 wherein the M^(III)N layer has a defect density ofapproximately 10⁴ defects per cm² or less.
 36. The method according toclaim 31 comprising the step of using the M^(III)N layer as a seed layerfor the growth of a bulk, single-crystal M^(III)N article.
 37. Themethod according to claim 36 wherein the M^(III)N article has the samecomposition as the M^(III)N columns.
 38. The method according to claim36 wherein the M^(III)N article has a different composition than theM^(III)N columns.
 39. The method according to claim 36 comprising thestep of growing the M^(III)N article by a technique selected from thegroup consisting of physical vapor deposition, sputtering, evaporation,sublimation, molecular beam epitaxy, atmospheric chemical vapordeposition, low pressure chemical vapor deposition, plasma-enhancedchemical vapor deposition, metallorganic chemical vapor deposition, andhydride vapor phase epitaxy.
 40. A single-crystal M^(III)N columnproduced according to the method of claim
 1. 41. The single-crystalM^(III)N column according to claim 40 wherein the column has a height ofapproximately 0.5 micron or greater and a lateral dimension ofapproximately 0.05 micron or greater.
 42. The single-crystal M^(III)Ncolumn according to claim 40 wherein the column has a defect density ofapproximately 10⁸ defects per cm² or less.
 43. The single-crystalM^(III)N column according to claim 40 wherein the column has a defectdensity of approximately 10⁴ defects per cm² or less.
 44. A method forproducing a single-crystal M^(III)N article comprising the steps of: (a)providing a template material having an epitaxial-initiating growthsurface; (b) sputtering a Group III metal target in a reaction chamberto produce a Group III metal source vapor; (c) introducing anitrogen-containing gas into the reaction chamber; (d) adjusting theIII/V ratio of Group III metal source vapor to nitrogen-containing gasto create a Group III metal-rich environment within the reaction chamberconducive to preferential column growth; (e) combining the Group IIImetal source vapor with the nitrogen-containing gas to produce areactant vapor species comprising Group III metal and nitrogen; (f)depositing the reactant vapor species on the growth surface to producesingle-crystal M^(III)N columns thereon; and (g) growing a bulk,single-crystal M^(III)N article on the M^(III)N columns.
 45. The methodaccording to claim 44 wherein the M^(III)N article is grown by atechnique selected from the group consisting of physical vapordeposition, sputtering, evaporation, sublimation, molecular beamepitaxy, atmospheric chemical vapor deposition, low pressure chemicalvapor deposition, plasma-enhanced chemical vapor deposition,metallorganic chemical vapor deposition, and hydride vapor phaseepitaxy.
 46. The method according to claim 44 wherein the M^(III)Narticle is grown in a form selected from the group consisting ofintrinsic M^(III)N, doped M^(III)N, and M^(III)N alloys and compoundscontaining greater than 50% M^(III) and N.
 47. The method according toclaim 44 wherein the M^(III)N article is grown at a growth rate ofapproximately 10 microns/hour or greater.
 48. The method according toclaim 44 comprising the step of rotating the M^(III)N article as theM^(III)N article grows.
 49. The method according to claim 44 wherein theM^(III)N article has a thickness ranging from approximately 1 micron togreater than 1 mm.
 50. The method according to claim 44 wherein theM^(III)N article has a diameter ranging from approximately 0.5 inch toapproximately 12 inches.
 51. The method according to claim 44 whereinthe M^(III)N article has the same composition as the M^(III)N columns.52. The method according to claim 44 wherein the M^(III)N article has adifferent composition than the M^(III)N columns.
 53. The methodaccording to claim 44 comprising the step of releasing the M^(III)Narticle to provide a free-standing, single-crystal M^(III)N article. 54.The method according to claim 53 wherein the M^(III)N article isreleased by performing a technique selected from the group consisting ofpolishing, chemomechanical polishing, laser-induced liftoff, cleaving,wet etching, and dry etching.
 55. The method according to claim 44comprising the step of cutting a wafer from the M^(III)N article. 56.The method according to claim 55 comprising the step of preparing asurface of the wafer for epitaxial growth thereon.
 57. The methodaccording to claim 56 wherein the step of preparing the surface of thewafer comprises polishing the surface of the wafer.
 58. The methodaccording to claim 55 comprising the step of depositing an epitaxiallayer on the wafer.
 59. The method according to claim 44 comprising thestep of forming a device on the M^(III)N article.
 60. A bulk,single-crystal M^(III)N article produced according to the method ofclaim
 44. 61. A single-crystal M^(III)N article produced according tothe method of claim 44 wherein the article has a diameter ofapproximately 0.5 inch to approximately 12 inches and a thickness ofapproximately 50 microns or greater.
 62. A single-crystal M^(III)Narticle produced according to the method of claim 44, wherein thearticle is in wafer form having a thickness ranging from approximately50 microns to approximately 1 mm.
 63. A single-crystal M^(III)N articleproduced according to the method of claim 44, wherein the article is inboule form having a diameter of approximately 2 inches or greater and athickness ranging from approximately 1 mm to greater than approximately100 mm.
 64. A single-crystal M^(III)N article produced according to themethod of claim 44 at a growth rate greater than approximately 10microns/hour.
 65. A method for producing a single-crystal M^(III)Narticle comprising the steps of: (a) providing a template materialhaving an epitaxial-initiating growth surface; (b) sputtering a GroupIII metal target in a plasma-enhanced reaction chamber to produce aGroup III metal source vapor; (c) introducing a nitrogen-containing gasinto the reaction chamber; (d) adjusting the III/V ratio of Group IIImetal source vapor to nitrogen-containing gas to create a Group IIImetal-rich environment within the reaction chamber conducive topreferential column growth; (e) combining the Group III metal sourcevapor with the nitrogen-containing gas to produce a reactant vaporspecies comprising Group III metal and nitrogen; (f) depositing thereactant vapor species on the growth surface to produce single-crystalM^(III)N columns thereon; and (g) readjusting the III/V ratio to createan environment within the reaction chamber conducive to columnarepitaxial overgrowth; (h) continuing to deposit the reactant species onthe growing M^(III)N columns, whereby uppermost regions of the M^(III)Ncolumns coalesce and grow to form a substantially continuous,single-crystal M^(III)N layer; and (i) growing a bulk, single-crystalM^(III)N article on the M^(III)N layer.
 66. The method according toclaim 65 wherein the step of readjusting the III/V ratio comprisesreadjusting the nitrogen concentration in the reaction chamber.
 67. Themethod according to claim 65 wherein the M^(III)N article is grown by atechnique selected from the group consisting of sputtering, molecularbeam epitaxy, atmospheric chemical vapor deposition, low pressurechemical vapor deposition, plasma-enhanced chemical vapor deposition,metallorganic chemical vapor deposition, and hydride vapor phaseepitaxy.
 68. The method according to claim 65 wherein the M^(III)Narticle is grown in a form selected from the group consisting ofintrinsic M^(III)N, doped M^(III)N, and M^(III)N alloys and compoundscontaining greater than 50% M^(III) and N.
 69. The method according toclaim 65 wherein the M^(III)N article is grown at a growth rate ofapproximately 10 microns/hour or greater.
 70. The method according toclaim 65 comprising the step of rotating the M^(III)N article as theM^(III)N article grows.
 71. The method according to claim 65 wherein theM^(III)N article has a thickness ranging from approximately 1 micron toapproximately 1 mm.
 72. The method according to claim 65 wherein theM^(III)N article has a diameter ranging from approximately 0.5 inch toapproximately 12 inches.
 73. The method according to claim 65 whereinthe M^(III)N article has the same composition as the M^(III)N columns.74. The method according to claim 65 wherein the M^(III)N article has adifferent composition than the M^(III) N columns.
 75. The methodaccording to claim 65 comprising the step of releasing the M^(III)Narticle to provide a free-standing, single-crystal M^(III)N article. 76.The method according to claim 75 wherein the M^(III)N article isreleased by performing a technique selected from the group consisting ofpolishing, chemomechanical polishing, laser-induced liftoff, wetetching, and dry etching.
 77. The method according to claim 65comprising the step of cutting a wafer from the M^(III)N article. 78.The method according to claim 77 comprising the step of preparing asurface of the wafer for epitaxial growth thereon.
 79. The methodaccording to claim 78 wherein the step of preparing the surface of thewafer comprises polishing the surface of the wafer.
 80. The methodaccording to claim 77 comprising the step of depositing an epitaxiallayer on the wafer.
 81. The method according to claim 65 comprising thestep of forming a device on the M^(III)N article.
 82. A bulk,single-crystal M^(III)N article produced according to the method ofclaim
 65. 83. A single-crystal M^(III)N article produced according tothe method of claim 65 wherein the article has a diameter ofapproximately 0.5 inch to approximately 12 inches and a thickness ofapproximately 50 microns or greater.
 84. A single-crystal M^(III)Narticle produced according to the method of claim 65, wherein thearticle is in wafer form having a thickness ranging from approximately50 microns to approximately 1 mm.
 85. A single-crystal M^(III)N articleproduced according to the method of claim 65, wherein the article is inboule form having a diameter of approximately 2 inches or greater and athickness ranging from approximately 1 mm to greater than approximately100 mm.
 86. A single-crystal M^(III)N article produced according to themethod of claim 65 at a growth rate greater than approximately 10microns/hour.
 87. A method for producing single-crystal M^(III)N columnscomprising the steps of: (a) providing a template material having anepitaxial-initiating growth surface; (b) using a sputtering apparatuscomprising a non-thermionic electron/plasma injector assembly disposedin a reaction chamber to produce a Group III metal source vapor from aGroup III metal target; (c) introducing a nitrogen-containing gas intothe reaction chamber; (d) adjusting the III/V ratio of Group III metalsource vapor to nitrogen to create a Group III metal-rich environmentwithin the reaction chamber conducive to preferential column growth; (e)reacting the Group III metal source vapor with the nitrogen-containinggas to produce a reactant vapor species comprising Group III metal andnitrogen; and (f) depositing the reactant vapor species on the growthsurface to produce single-crystal M^(III)N columns thereon.
 88. Themethod according to claim 87 wherein the injector assembly comprises aplurality of hollow cathode injectors disposed in fluid communicationwith a gas source, each injector including an orifice communicating witha sputtering chamber.
 89. The method according to claim 87 wherein theinjector assembly comprises: (a) a main body having a generally annularorientation with respect to a central axis and including a process gassection and a cooling section, the process gas section defining aprocess gas chamber and the cooling section defining a heat transferfluid reservoir; and (b) a plurality of gas nozzles removably disposedin the main body in a radial orientation with respect to the centralaxis and in heat transferring relation to the heat transfer fluidreservoir, each gas nozzle providing fluid communication between theprocess gas chamber and a region exterior to the main body.
 90. Themethod according to claim 87 comprising the step of growing a bulk,single-crystal M^(III)N article on the M^(III)N columns.
 91. The methodaccording to claim 90 comprising the step of removing the templatematerial, thereby providing a free-standing, single-crystal M^(III)Narticle.
 92. The method according to claim 90 wherein the step ofgrowing the M^(III)N article comprises the steps of: (a) readjusting theIII/V ratio to create an environment within the reaction chamberconducive to columnar epitaxial overgrowth; (b) continuing to depositthe reactant species on the growing M^(III)N columns, whereby uppermostregions of the M^(III)N columns coalesce and grow to form asubstantially continuous, single-crystal M^(III)N layer; and (c) growingthe M^(III)N article on the M^(III)N layer.
 93. The method according toclaim 92 wherein the step of readjusting the III/V ratio comprisesincreasing the nitrogen concentration in the reaction chamber.